The present invention relates to a device for detecting the focal plane in an optical apparatus, such as a camera.
In focal plane detecting means it is known to utilize the fact that the contrast of an image assumes its maximum in the focal plane i.e., the difference between bright and dark portions is greatest. As an expedient for obtaining a signal corresponding to the contrast the image surface is scanned by vibrating an element and a slit using an excitor or by rotating a disc formed with a slit on the shaft of a motor.
However, the method for scanning the image surface by means of an excitor carries out a high speed scan, and hence, when vibration is applied at a high frequency, the exciting amplitude decreases with such vibration. From this, the above-mentioned method poses a drawback such that a sufficient contrast signal may not be obtained unless an object has a high space frequency. Also, according to the method wherein the image surface is scanned by rotation of a slotted disc, the range to be scanned may suitably be varied by varying position from the center of rotation to the slot. However there still remains the problem of how to stabilize the signal from a photoconductive element disposed rearwardly of the slot. For example, in Japanese Patent Publication No. 43-25723, such a signal is obtained by a slide ring and a brush, but the contact resistance therebetween always varies during rotation of the motor shaft, with the result that it is difficult to stably obtain a minute output signal. Further, since the motor shaft must have various mechanisms mounted thereon, the motor requires a high degree of torque in order to achieve a high speed of rotation. This is a disadvantage in terms of electric power consumed.